1 Field of the Invention
The present invention relates to an optical power equalizer in a wavelength division multiplexing(WDM) optical communication system, and more particularly, to an optical power equalizer in a WDM optical communication system which can tune an optical power down to a desired extent and a variable optical attenuator for use therein.
2. Background of the Related Art
Having overcome a level in which a light is merely taken as a transmission medium in place of electricity, currently the optical communication has come to a level in which an optical signal is converted into an electrical signal at a final reception terminal after amplifying the optical signal for a longer distance transmission or processing the optical signal while maintaining a form of the optical signal without converting the optical signal into an electrical signal. And, recently, the wavelength division multiplexing technique is rapidly developed and spread for increasing a transmission capacity, in which an individual light source is provided for each of wavelengths in an optical signal, multiplexed before transmission, demultiplexed at a reception terminal, split into the individual wavelengths, thereby receiving the optical signal. And, sometimes, an optical amplifier is provided in the middle of the transmission for a long distance transmission. An optical communication network is sometimes provided with an add-drop function in which portions of the optical signal are allocated to required places and optical signals from the places are added thereto.
FIG. 1 schematically illustrates a related art WDM add-drop system, in which an optical signal transmitted through an optical amplifier 10 is demultiplexed through a demultiplexer 11 into individual wavelengths .lambda..sub.1,.lambda..sub.2,- - -,.lambda..sub.n, add-drops are made for the individual wavelengths through optical switches 12, multiplexed through a multiplexer 14, amplified through an optical amplifier, and transmitted through one optical fiber. In this instance, as shown in FIG. 2A, the optical signal before the optical signal is multiplexed exhibits a non-uniform state between different wavelengths. Though there are many reasons which cause this non-uniform state, this non-uniform state typically comes from composite factors, such as differences of amplification gains between the wavelengths when the optical signal is amplified through the optical amplifier, differences in the wave splitting performances of the multiplexer 11 between the individual wavelengths, non-uniform switching characteristics of the optical switches 12, which cause the non-uniform state between the individual wavelengths in the optical signal at an input terminal of the multiplexer 14 so serious that a signal characteristic is deteriorated too much unable to make a long distance communication. Consequently, it is required to make powers of the optical signals uniform before the multiplexing, for which the optical attenuators 13 as shown in FIG. 1 are provided, owing to which outputs of the individual wavelengths are made uniform as shown in FIG. 2B. The optical attenuator, generally used currently with optical fibers handled manually, is not responsive to continuous signal changes since the system is fixed once adjusted initially. Therefore, a variable optical attenuator is required, which can be used continuously while monitoring an optical signal. Such a variable optical attenuator is disclosed, for example, in U.S. Pat. No. 4,644,145. Though the U.S. Pat No. 4,644,145 discloses variable optical attenuators used in optical waveguides for compensating variations of signals received at an optical receiver, with the object of the U.S. Pat. 4,644,145 lying on improvement of a receiver performance merely by making sizes of signals at a reception terminal uniform, it is very difficult to employ the optical attenuator in the WDM optical communication because the optical attenuator is one used before the optical communication system technology is matured, in which each of fast optical signals received in succession is attenuated at a fast speed for compensating time basis non-uniformity of the optical signals. That is, for modulation of the fast optical signals, though the variable optical attenuator in the U.S. Pat. No. 4,644,145 uses LiNbO.sub.3 as a substrate for utilizing an electrooptical effect or GaAs, a semiconductor material, a device using LiNbO.sub.3 as a substrate is expensive despite fast modulation is allowed, and not suitable for insertion in the middle of transmission for use, not in a few GHz range of fast modulation, but in the add-drop multiplexing system at a few kHz range of frequency because of a low coupling efficiency of the optical fiber and a device using GaAs, a semiconductor material, can not be put into practical use because of a further lower coupling efficiency and expensive material. And, the related art variable optical attenuator has a micrometer which uses an optical fiber for manual mechanical manipulation of the micrometer, or a micrometer fitted with a motor for electric control. However, the related art variable optical attenuator with a micrometer fitted with a motor, not only has many problems in actual application to the WDM optical communication system because it has a large power consumption, high driving voltage over 10V, relatively bulky, and a slow tuning speed of 50-1,400 msec, but also has a problem of high cost because the device is not suitable for mass production in view of the device structure.